The long-term objective of this research is to advance our understanding of neural mechanisms underlying the encoding and processing of acoustic signals in the caudal parts of the auditory pathways, i.e., the auditory nerve (AN) and the cochlear nucleus (CN). Our specific aims are: (1) to investigate neural basis for behavioral intensity discrimination for pure tones presented in quiet and in masking noise by examining spatial profiles of discriminability measure d(e) defined in the manner of psychophysical signal detection theory, of various spontaneous rate (SR) populations of AN fibers; (2) to investigate neural basis of AN populations for behavioral frequency discrimination for pure tones by examining spatial profiles of discriminability measure d(e) in a manner similar to the above intensity discrimination study; (3) to conduct population studies of responses of AN fibers and several classes of CN neurons to complex signals (vowels and consonant-vowel syllables) presented in quiet and in masking noise and to delineate the transformations of spectro-temporally complex signal representation taking place between the AN and each of several classes of CN neurons; (4) to investigate CN neurons' behaviors of encoding amplitude-modulated tones in quiet and in masking noise and to compare them with those of AN fibers; distinct behaviors of "chopper" subclass neurons and "pause-build" neurons will be elucidated and they will be compared with predictions of CN neuronal models; (5) to develop neurobiologically plausible models for "stellate", "bushy", and "fusiform" cells of the CN based on the hypothesis that the membrane of different cell types has either voltage-independent conductance or distinct voltage-dependent nonlinear conductance; the models will attempt to reproduce cell-specific current-voltage and spike discharge characteristics in response to intracellular currents or to synaptic inputs; and (6) to develop neurobiologically plausible models for several classes of CN neurons exhibiting band-pass modulation transfer functions (MTFs), e.g., "pause- build" and sustained "chopper" neurons, and other CN neurons exhibiting low-pass MTFs, e.g., transient "chopper" neurons; the model predictions will bc closely compared with actual neural responses. The experimental methods will include obtaining population responses of auditory nerve fibers and cochlear nucleus neurons in unanesthetized decerebrate cats by sequentially recording from a large number of single neurons/fibers while applying an identical set of acoustic stimuli. The neural modeling studies will use digital computer simulation of neurons represented by an equivalent circuit for the cell membrane and a spike generation mechanism. The scientific information and theories to be obtained from this proposed research should be valuable in the long run to advance our understanding of the normal function of the auditory system and how the hearing capacities of disordered human auditory systems are degraded and, ultimately, in developing effective prostheses that can improve the hearing capacity of people with sensorineural hearing loss.